1. Field of Invention
The invention relates to drive coupled, dual motor, reversible hydraulic drive systems, and in particular, to an anti-cavitation hydraulic manifold hydraulically coupling the hydraulic inputs and outputs of the respective motors to a reversible source of hydraulic power.
2. Description of Prior Art
In U.S. Pat. Nos. 5,184,357 & 5,327,590 both entitled AUTOMATIC SWIMMING POOL COVER WITH A DUAL HYDRAULIC DRIVE SYSTEM respectively issuing Feb. 9, 1993 and Jul. 12, 1994, the Applicant describes a dual hydraulic drive system for automatic swimming pool cover systems in which a first hydraulic drive provides torque for both resisting cover drum rotation during pool cover extension across a swimming pool and rotating the cover drum for cover retraction, while a separate second hydraulic drive provides torque for both rotating the cable reels for cover extension and resisting cable reel rotation during cover retraction from across the swimming pool. The drives of the reversible hydraulic motors are mechanically coupled by interconnecting cables and the cover such that each functions as both a motor and a pump in such hydraulic drive systems, i.e., when one motor is hydraulically driven to provide torque, the other motor hydraulically responds as a pump. A manifold hydraulically couples the source of hydraulic power to the respective inputs of the driving motors and hydraulically couples the exhaust from the driving motor to input for the pumping motor. And, as explained by the applicant in U.S. Pat. No. 5,327,590 [Col. 8, 11. 25-40], when the hydraulic liquid exhausting from the driving motor exceeds the demand for liquid of the pumping motor, the pumping motor can be driven or rotated at a faster rate causing it to unwind a cable reel or cover drum at a rate faster than the driving motor winds the cover or cable with a consequent loss of resistance tension in the mechanical components of the system coupling the drives of the motors. On the other hand, when liquid supply from the driving motor is not sufficient, the pumping motor cavitates which causes sudden and erratic decreases in torque resistance at the drive shaft of the pumping motor with a consequent erratic decreases in tension/load on the mechanical components coupling the drives of the motors.
In the referenced patents, the Applicant teaches that loss of tension due to oversupply can be alleviated by locating check valves which functionally open to allow hydraulic liquid to flow from a common line hydraulically coupling output of the driving motor to the input of the pumping motor when pressure in the common line exceeds that in the output line of the pumping motor. U.S. Pat. No. 5,327,590 [Col. 8, 11. 41-45] Addressing the problem of erratic losses of resistance tension due to cavitation of the pumping motor proved more troublesome.
Initially, the applicant found that cavitation could be prevented by using a novel arrangement of conventional check valves and pressure relief valves [See U.S. Pat. No. 5,184,357, Claims 3 & 4,] which allowed the pumping motor to `pump` or draw make-up liquid from the reservoir. That arrangement worked well provided the hydraulic liquid reservoir was hydraulically proximate to the coupled motors, however, it was discovered that when the source of hydraulic power and reservoir were located hydraulically remote from the coupled motors, the negative pressure or suction of the pumping motor was not sufficient to draw make-up liquid from the reservoir and intermittent cavitation resulted. The problem was particularly sever for long cover systems where the differential in wind-up diameters and consequent surface speeds of cable reel and cover drum were significant.
Subsequently, the applicant developed a novel manifold located hydraulically proximate to the coupled motors to preclude cavitation which included pilot operated check/lock valves which opened to hydraulically coupling the input or common bypass line to the exhaust input/output line of the pumping motor. [See U.S. Pat. No. 5,327,590, Claims 5-7] This allowed the pumping motor to draw made-up liquid from its own output, i.e., allowed the hydraulic liquid to loop back to the inlet side of the pumping motor. In this subsequent patent, applicant also pointed out that for optimal operation, it would be necessary, under some circumstances, to increase the volume of the common bypass line hydraulically coupling the motors such that it functioned as an reservoir cavity in order to preclude cavitation particularly at start up. [See U.S. Pat. No. 5,327,590, Col. 10, 11. 29-46.]
However, pilot operated check/lock valves proved to unsatisfactory when quickly reversing the drive, i.e., the status of the driving and pumping motors were switched. In particular, the change in hydraulic pressure was not promptly reflected in the respective pilot lines which momentarily caused one valve to open while allowing the other to remain open enabling the hydraulic liquid to circulate temporarily bypassing the coupled motors, i.e., to free-wheel. The inability to rapidly reverse the drive mechanism is a disadvantage and possibly hazardous because, for example, with automatic pool cover systems, it is desirable to be able to rapidly reverse cover extension to free a child or person trapped, for example, between the leading edge of the cover and an obstruction, or worse yet, beneath the cover.
Another characteristic of the drive coupled, dual motor, reversible hydraulic drive systems taught by the Applicant in his patents is that the respective drives of the respective motors inherently lock and do not rotate when the hydraulic liquid is precluded from circulating. Such locking has particular advantages as the Applicant points out in U.S. Pat. No. 5,327,590 [Col. 8, 1. 66-Col. 9, 1. 59]. However, such locking also has disadvantages in that, unless decoupling mechanisms such as ratchets are utilized to decouple rotation of the motor drive from that of a cable reel or cover drum, it is necessary disconnect one or more of the hydraulic lines to allow the locked motor to free-wheel in order, for example, to unwind a cable or cover from a reel or drum. In particular, often it is not desirable or even feasible to power-up the drive system in order to unwind or wind an interconnecting cable/cover mechanically coupling the motor drives. Also disconnecting the hydraulic lines means a probable contaminating spill and loss of hydraulic liquid.